The present invention relates generally to encapsulated devices, and more particularly to barriers for encapsulation, and to methods for making layers for said barriers.
Many devices are subject to degradation caused by permeation of environmental gases or liquids, such as oxygen and water vapor in the atmosphere or chemicals used in the processing of the electronic product. Some devices are often encapsulated in order to prevent degradation.
Various types of encapsulated devices are known. For example, U.S. Pat. No. 6,268,695, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making,” issued Jul. 31, 2001; U.S. Pat. No. 6,522,067, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making,” issued Feb. 18, 2003; and U.S. Pat. No. 6,570,325, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making”, issued May 27, 2003, all of which are incorporated herein by reference, describe encapsulated organic light emitting devices (OLEDs). U.S. Pat. No. 6,573,652, entitled “Encapsulated Display Devices”, issued Jun. 3, 2003, which is incorporated herein by reference, describes encapsulated liquid crystal displays (LCDs), light emitting diodes (LEDs), light emitting polymers (LEPs), electronic signage using electrophoretic inks, electroluminescent devices (EDs), and phosphorescent devices. U.S. Pat. No. 6,548,912, entitled “Semiconductor Passivation Using Barrier Coatings,” issued Apr. 15, 2003, which is incorporated herein by reference, describes encapsulated microelectronic devices, including integrated circuits, charge coupled devices, light emitting diodes, light emitting polymers, organic light emitting devices, metal sensor pads, micro-disk lasers, electrochromic devices, photochromic devices, microelectromechanical systems, and solar cells.
One method of making encapsulated devices involves depositing barrier stacks adjacent to one or both sides of the device. The barrier stacks typically include at least one barrier layer and at least one decoupling layer. There could be one decoupling layer and one barrier layer, there could be multiple decoupling layers on one side of one or more barrier layers, or there could be one or more decoupling layers on both sides of one or more barrier layers. The important feature is that the barrier stack has at least one decoupling layer and at least one barrier layer.
One embodiment of an encapsulated display device is shown in FIG. 1. The encapsulated display device 100 includes a substrate 105, a display device 110, and a barrier stack 115. The barrier stack 115 includes a barrier layer 120 and a decoupling layer 125. The barrier stack 115 encapsulates the display device 110, preventing environmental oxygen and water vapor from degrading the display device.
The barrier layers and decoupling layers in the barrier stack can be made of the same material or of a different material. The barrier layers are typically about 100-1,000 Å thick, and the decoupling layers are typically about 1,000 Å thick.
Although only one barrier stack is shown in FIG. 1, the number of barrier stacks is not limited. The number of barrier stacks needed depends on the level of water vapor and oxygen permeation resistance needed for the particular application. One or two barrier stacks should provide sufficient barrier properties for many applications, while three for four may be sufficient for most. The most stringent applications may require five or more barrier stacks. Another situation in which multiple barrier stacks may be required is where the thickness of the decoupling layer needs to be limited to limit the stress induced by the polymer shrinkage, such as with passive matrix devices.
The barrier layers can be deposited using a vacuum process, such as sputtering, chemical vapor deposition, plasma enhanced chemical vapor deposition, evaporation, sublimation, electron cyclotron resonance-plasma enhanced vapor deposition (ECR-PECVD), and combinations thereof.
Suitable barrier materials include, but are not limited to, metals, metal oxides, metal nitrides, metal carbides, metal oxynitrides, metal oxyborides, and combinations thereof.
We have found that some of the devices being encapsulated have been damaged by the plasma used in depositing the barrier and/or decoupling layers. Plasma damage has occurred when a substrate with an environmentally sensitive device on it, such as an OLED, is encapsulated with a multi-layer barrier stack in which a plasma based and/or assisted process is used to deposit a barrier layer or decoupling layer. For example, plasma damage has occurred when reactively sputtering a barrier layer of AlOx under conditions suitable for achieving barrier properties, sputtering a barrier layer of AlOx onto the top surface of an environmentally sensitive device, and/or sputtering a barrier layer of AlOx on a vacuum deposited, acrylate based polymeric layer.
Plasma damage involves a negative impact on the electrical and/or luminescent characteristics of a device resulting from encapsulation. The effects will vary by the type of device, the manufacturer of the device, and the wavelength of the light emitted. It is important to note that plasma damage is dependent on the design of the device to be encapsulated. For example, OLEDs made by some manufacturers show little to no plasma damage, while OLEDs made by other manufacturers show significant plasma damage under the same deposition conditions. This suggests that there are features within the device that affect its sensitivity to plasma exposure.
One way to detect plasma damage is to observe changes in the I-V-L characteristics of the device.
The decoupling layers can be deposited using a vacuum process, such as flash evaporation with in situ polymerization under vacuum, or plasma deposition and polymerization, or atmospheric processes, such as spin coating, ink jet printing, screen printing, or spraying. U.S. Pat. Nos. 4,842,893, 4,954,371, and 5,032,461, which are incorporated herein by reference, describe a method of flash evaporation and polymerization. Suitable materials for the decoupling layer, include, but are not limited to, organic polymers, inorganic polymers, organometallic polymers, hybrid organic/inorganic polymer systems, and silicates.
It was believed that the primary contribution of the decoupling layer to the barrier performance of a barrier stack was that it prevented defects in one barrier layer from propagating into another. By alternating barrier layers and decoupling layers, defects in one layer tend to be isolated and are not carried forward in the next layer. This creates a longer and more tortuous path for contaminants, such as oxygen and water vapor.
For example, U.S. Pat. No. 5,681,666 (Treger) discusses the importance of the layers of inorganic material being separated by organic material to avoid crack and defect propagation in the inorganic material. Treger indicated that cracks, pinholes, or other defects in an inorganic layer tends to be carried into the next inorganic material layer if the next inorganic material layer is deposited directly onto the first layer of inorganic material with no intervening layer of organic material between the two inorganic layers. According to Treger, this phenomenon significantly reduces the usefulness of the composite as a moisture barrier, since the defects often propagate through all of the inorganic layers if no organic layer is interposed between them.
A similar phenomenon sometimes occurs with respect to organic layers. Thus, a macroscopic or microscopic pinhole, inclusion of a dust particle, etc., can occur during the deposition of the organic layer, and this provides an easy path for water vapor transmission. By depositing alternating organic and inorganic layers, the defects in any particular layer do not tend to propagate into the next layer. This provides a much longer and more tortuous path for the water vapor to go through, even to such an extent that the net result is as though such defects do not exist.
From technical view point, thinner layers and more layers provide more resistance to the transmission of water vapor through the composite. However, the cost of the moisture barrier increases with each layer that is deposited. Also, if the layers are too thin, there will be voids of incomplete coverage in the layers, and this will increase the permeability of the composite.
This thinking is also reflected in “Mechanisms of vapor permeation through multilayer barrier films: Lag time versus equilibrium permeation,” G. L. Graff, et al., Journal of Applied Physics, Vol. 96, No. 4, p. 1840 (Aug. 15, 2004), which is incorporated herein by reference. Graff et al. explain that permeation through single and multilayer vapor barriers is controlled by defects, and that defect size and spatial density are the critical parameters for defining barrier performance. Although the long apparent diffusion path caused by separating low defect density inorganic layers from each other with polymer layers significantly increases lag times, the decrease in steady-state flux is much less significant. The increased lag time is primarily responsible for the improvement in barrier performance as additional barrier stacks are added.
Graff et al. suggest that lowering the diffusivity and solubility of the polymer layers will improve the barrier performance. This can be accomplished by polymer selection (hydrophobic moieties or organic/inorganic copolymers), physical modifications (such as ion bombardment or crosslinking), or chemical modification (reactive etch or plasma surface treatment). However, they indicate that the range of improvement that is possible with the polymer layer may be insignificant relative to the improvement of the inorganic layer because the effective diffusion of the inorganic layer is at least four orders of magnitude lower than that of the polymer layers.
There was an underlying assumption that the permeating species reaching and then directly degrading the OLED is the only factor in barrier failure.
It is known that plasma treatments can modify the properties of polymers. Several patents disclose the use of plasma treatment to improve properties for a multi-layer barrier on a substrate. U.S. Pat. No. 6,083,628 discloses plasma treatment of polymeric film substrates and polymeric layers from acrylates deposited using a flash evaporation process as a means of improving properties. U.S. Pat. No. 5,440,466 similarly discusses plasma treatment of substrates and acrylate layers to improve properties. On the other hand, it is known that in some cases plasma and/or radiation exposure degrades the functional properties of polymers. Thus, there is a need for improved polymeric decoupling layers for barrier stacks which are more compatible with all of the available deposition technologies.